Polyacetal resins are well known engineering resins due to their balanced mechanical properties. In this regard, polyacetal resins usually exhibit desirable physical properties in terms of their frictional abrasive characteristics, electrical properties and chemical and heat resistance properties. As a result, polyacetal resins are being used extensively as a material from which parts for automobiles, electrical and electronic devices and the like are made. The properties that are, however, required of polyacetal resins in such end-use applications are continually in need of improvement due to the special environments of use.
The sliding characteristics of polyacetal resins are examples of properties which sometimes need improvement. In this regard, the sliding characteristics of polyacetal resin involve not only its anti-friction characteristics (as determined by coefficients of friction) and abrasion-resistance properties, but also anti-frictional noise (i.e. squeaking) characteristics. All such properties are important when polyacetal resins are used as sliding members--that is, a component or part having a surface which bears against a surface of another component or part. It can be appreciated, therefore, that all such characteristics--that is, anti-friction and anti-frictional noise characteristics, in addition to abrasion resistance--are important when polyacetal resins are employed as wear parts.
Several techniques have been employed in the past to improve the sliding characteristics of polyacetal resins. For example, it has been suggested that other resins, such as fluororesins or polyolefin resins, or lubricating oils, such as fatty acids, fatty acid esters, silicone oils or mineral oils may be added to polyacetal base resins to improve the sliding characteristics thereof. Some improvements, for example, improvements in anti-frictional noise characteristics, are obtained by adding fluororesins or polyolefin resins to a polyacetal base resin. However, these resins typically exhibit poor compatability with the polyacetal base resin and therefore tend to delaminate on the surface of molded articles formed from such resin compositions. In addition, the poor compatability of the resins in the composition may form mold deposits during injection molding of a part formed thereof.
Similarly, the addition of lubricating oils is typically effective to reduce coefficients of friction and improve abrasion resistance of molded articles. However, the addition of lubricating oils is usually inadequate to improve the anti-frictional noise characteristics of the wear part. Several other problems also exist when lubricating oils are added to polyacetal resins, such as extrusion difficulties and "bleeding" of the lubricating oil onto the surface of molded parts during use.
Thus, although some improvements have been achieved by means of the prior art techniques described above, further improvements in terms of anti-friction, anti-frictional noise and abrasion-resistance characteristics for sliding members, while maintaining and/or balancing the otherwise desirable mechanical/physical properties of polyacetal resins, have been needed.
Broadly, the present invention relates to polyacetal resin molding compositions and molded parts therefrom that exhibit well balanced anti-friction, anti-frictional noise and abrasion-resistance characteristics, moldability, little (if any) delamination problems, satisfactory surface hardness and machinability. The compositions of the present invention are obtained by blending a specific graft copolymer, a lubricant and an inorganic powder having specific particle size limitations with a polyacetal base resin.
More specifically, the present invention is embodied in polyacetal resin molding compositions which include 100 parts by weight of a polyacetal base resin, between 0.5 to 40 parts by weight of a branched or cross-linked graft copolymer which is the graft-reaction product of (a) an olefin polymer, and (b) a vinyl or ether polymer, between 0.1 to 20 parts by weight of a lubricant, and between 0.5 to 30 parts by weight of an inorganic powder having a mean particle diameter of 50 .mu.m or less, and wherein at least 95% of the particles have a particle diameter of 100 .mu.m or less of at least 95%.
Further aspects and advantages of the present invention will become more clear after careful consideration is given to the detailed description of the preferred exemplary embodiments thereof which follow.